Terephthalic acid (TPA) is one of the basic building blocks in the production of linear polyester resins used in the manufacture of polyester films, packaging materials and bottles. TPA used in the manufacture of such polyesters resins must meet certain minimum purity requirements. The purified condition of terephthalic acid refers primarily to the absence of significant concentrations of 4-carboxybenzaldehyde (4-CBA) and p-toluic acid that are present in significant quantities in the crude commercially-available grades of terephthalic acid. Both CBA and toluic acid are partial oxidation products formed in the manufacture of TPA by the catalytic oxidation of p-xylene. The purified form also refers to the absence of color bodies that impart a characteristic yellow hue to the crude material. The color bodies are aromatic compounds having the structures of benzils, fluorenones, and/or anthraquinones. 4-CBA and p-toluic acid are particularly detrimental to the polymerization process as they act as chain terminators during the condensation reaction between terephthalic acid and ethylene glycol in the production of poly(ethylene terephthalate) (PET).
To obtain purified terephthalic acid (PTA) from crude TPA, the 4-CBA and the color bodies are hydrogenated, the 4-CBA to p-toluic acid and the color bodies to compounds that are colorless solids. Typically, crude terephthalic acid dissolved in a solvent such as water is subjected to a liquid phase hydrogenation of the impurities in the presence of an immobilized or fixed bed catalyst. The 4-CBA is converted to p-toluic acid in high yields.
The hydrogenation process proceeds at elevated temperatures of between 250xc2x0 C. and 280xc2x0 C. using a partial pressure of hydrogen in the range 0.5 to 20 bars absolute-bara (0.05 to 2.0 Mpa). The concentration of the TPA in aqueous TPA solutions fed to the hydrogenation reactor typically is in the range of about 15 to 30 weight percent. The hydrogenated product stream normally is passed to a series of crystallization units in which purified terephthalic acid (PTA) is crystallized from solution in a crystalline form that can be readily filtered and dried.
The staged equilibrium crystallization technique is described in U.S. Pat. No. 3,452,088 which discloses the controlled evaporation or flashing of solvent by back-pressure regulation in multiple stages to control the rate at which the hydrogenation product stream is crystallized. U.S. Pat. No. 3,542,088 discloses that shock cooling of the post-hydrogenation stream to temperatures below 165xc2x0 C. should be avoided since shock or sudden cooling promotes the co-precipitation of other impurities, particularly p-toluic acid, which contaminate the purified TPA product. This caution is repeated in more general terms in U.S. Pat. No. 3,931,305, which states that xe2x80x9cSuch contamination phenomenon is somewhat anomalous because, in spite of the fact that there is retained more than enough solvent water to prevent saturation or supersaturation with respect to p-toluic acid, p-toluic acid nevertheless comes out of solution.xe2x80x9d U.S. Pat. No. 3,452,088 suggests that the contamination phenomenon is in some way dependent on the rate of crystallization and the final temperature of crystallization and product separation and not solely on p-toluic acid concentration in the solution.xe2x80x9d U.S. Pat. No. 3,931,305 concludes that the primary factor determining p-toluic acid concentration in the final TPA product is the lowest temperature to which the post-hydrogenation solution is flashed. It is less a function of the rate at which it is cooled to this temperature. It was determined that a final filtration temperature of between 121 and 149xc2x0 C. is desired to obtain a p-toluic acid concentration of less than 150 ppm in the final TPA product when the crude material has a concentration from 500 ppm to 6,000 ppm.
U.S. Pat. No. 3,931,305 discloses that in a system wherein TPA is crystallized in a train of series-connected crystallizers, the temperature dependent precipitation of TPA becomes critical below a temperature between 160 and 182xc2x0 C. The ""305 patent thus recommends that the majority of the TPA be crystallized before this threshold is reached to minimize contamination with p-toluic acid. More specifically, the ""305 patent discloses the crystallization of 75-95% of the originally dissolved TPA in substantially equal portions in the first two crystallization zones at a temperature of 160 to 182xc2x0 C. and thereafter crystallizing the remaining 5-25% of the originally dissolved TPA in decreasing incremental portions.
Another limitation on the recovery of TPA substantially free of p-toluic acid is set by the lowest processing temperature at which the TPA solids can be separated from the crystallization mother liquor. Based on the above-cited patent literature, this temperature is above the normal boiling temperature of the water solvent. Hence, any process for separating the TPA solids from the crystallization mother liquor must be conducted at superatmospheric pressures. Such a processing limitation requires the separation equipment to have a more robust construction than its atmospheric or near atmospheric pressure counterparts. Hence, from the standpoint of capital investment cost, use of atmospheric or near atmospheric pressure separation equipment is desirable.
The present invention provides a process for the recovery of purified TPA product from a hydrogenation product obtained by the hydrogenation of a solution of crude TPA using a sequence of series-connected crystallizers. The present invention provides a process for the recovery of crystalline terephthalic acid containing less than about 150 parts per million by weight (ppmw) p-toluic acid, based on the weight of the terephthalic acid, by the steps comprising:
(1) providing a solution containing about 10 to 35 weight percent dissolved terephthalic acid having dissolved therein about 150 to 1100 ppmw p-toluic acid, based on the weight of the terephthalic acid present, and having a temperature of about 260 to 320xc2x0 C. at a pressure sufficient to maintain the solvent in the liquid phase;
(2) feeding the solution of step (1) to a crystallization zone comprising a plurality of series-connected crystallizers wherein the solution is subjected to rate-controlled evaporative cooling by sequential reduction in pressure and temperature to cause crystallization of terephthalic acid, wherein the pressure of the solution at the end of the crystallization zone is about ambient pressure or less;
(3) condensing solvent evaporated from the crystallizers and returning the condensed solvent to the crystallization zone at a point subsequent to the crystallizer from which it was obtained; and
(4) recovering solid, crystalline terephthalic acid containing less than about 150 parts ppmw p-toluic acid, based on the weight of the terephthalic acid, by liquid-solid separation at ambient pressure.
According to our invention, solvent evaporated from at least one of the crystallizers constituting the crystallization zone is condensed and recycled to one of the subsequent crystallizer stages. The advantages provided by our novel process include the recovery of terephthalic acid in an improved crystalline form with less xe2x80x9cfinesxe2x80x9d, i.e., small crystals or particles of TPA, which can cause problems in the handling and conveying of the TPA. Another advantage is the product recovery at ambient or approximately ambient pressure.
To obtain the same recovery of TPA per stage as demonstrated in U.S. Pat. No. 3,931,305, the temperatures can be staged much closer to each other in the process of the present invention, thus allowing shock cooling of the post-hydrogenation stream to be minimized at the temperatures where most of the TPA is crystallized from solution. The corollary of this statement is also true: that at the temperatures demonstrated in U.S. Pat. No. 3,931,305 more of the TPA will crystallize from solution at the stated unit temperatures when the system is operated as described in this invention. For a given residence time and production rate, the volume of the crystallizers required by the process described herein is much smaller than the volume required by known process as the initial TPA concentration in solution is much higher while still targeting the same suspended solids content in the final product stream. The smaller volume that is required of the crystallizers results in a significant cost saving. The crystallization of TPA at higher temperatures is reported to cause less p-toluic acid to be co-crystallized with the TPA. This further contrasts the present process from the process and the critical temperature range described in U.S. Pat. No. 3,931,305.
Recycle of condensed solvent directly back into the crystallizer stage from which it evaporated, commonly known as total reflux, does not meet the requirements of the present invention since such a reflux stream is acting as an additional feed stream diluting the TPA laden feed stream. This increase in total feed material requires an enlargement of the vessel volume to maintain a given residence time which may not be desired.